Compressed air systems are widely used to operate a number of devices, and find particular use in braking systems for heavy vehicles. These systems include an air compressor which compresses ambient air and charges a storage reservoir. The compressor is engine operated and a governor or controller, responsive to the reservoir pressure, selectively enables or disables the compressor as needed. Air dryers commonly employ a drying material or desiccant through which air passes to remove moisture and limit problems associated with moisture entering brake system components. Moisture is adsorbed by the desiccant and removed from the compressed air before it is transported to the storage reservoir. In these known systems, periodic or cyclic purging of the air dryers is required in order to purge or regenerate the desiccant. A typical air brake drying system in present day trucks functions as follows: Ambient air is drawn in through the engine air filter, which removes particulate contaminants. The air then enters the suction side of a turbocharger or super charger, which boosts the pressure to about 103 kPa (15 psig). An intercooler is used to cool the air. Most of the boosted air then enters the engine intake, but a portion of it (typically 12 SCFM, but varying according to the engine speed) flows to the air compressor. The compressor increases the pressure from about 103 kPa (15 psig) to the system pressure of about 793 to 896 kPa (115 to 130 psig). The compressed air leaving the compressor has been heated by the compression process and as a result is capable of holding a relatively large amount of water. As the air cools in the system, it loses its capability to retain as much moisture, which results in the formation of liquid water in the system. This moisture and other contaminants can be detrimental to the system in that freezing or sludge buildup can occur. Excessive moisture can also wash away needed lubricants in the valves and other air system components. Most commercial trucks, tractors and buses use some form of air dryer to remove the unwanted moisture and contaminants from the air brake system. The most common air dryers in use today are of the desiccant variety. In a desiccant air dryer, some water condenses before it reaches the desiccant and remaining water molecules are adsorbed by the desiccant material within the air dryer. The moisture is retained by the desiccant material until a quantity of dry expanded air is flowed over the desiccant to regenerate the desiccant. If there is too long a period before regeneration occurs, then the desiccant will become saturated and the air dryer effectively stops functioning. The type of air dryer employed is a pressure swing dryer. After being dried in the pressure swing dryer, the compressed air flows to a supply tank and system reservoirs.
Most vehicles have a single cartridge air dryer. This dryer works intermittently along with the compressor cycles. During the compressor load cycle (when the compressor is running), the single dryer is adsorbing moisture from compressed air. During the compressor unload cycle, when it is idle, the dryer is taken off of adsorption and the depressurization and desorption purge steps occur. The purge gas can come from one of three places. The purge gas can be stored during adsorption in a separate pressure tank external to the dryer. It can be stored in a volume integral to the dryer or desiccant cartridge itself. It can come from the supply reservoir of the compressed air system.
In those systems that have a twin cartridge or two bed air dryer, as the compressor is working, one bed is on adsorption and the other bed is on desorption. The two beds periodically switch from adsorption to desorption and back again. The twin bed dryer has the ability to operate continuously in a pressure swing adsorption cycle, although flow through the dryer stops during the compressor unload cycle, when the compressor is idle. Compressed air enters the dryer bed on adsorption and the dried air exits the bed. A portion of the dried air is used for continuously purging the bed on regeneration. The balance of the air is supplied to the brakes and other pneumatic accessories. The cycle may be timed, so that after a given amount of air has been dried, the beds switch or the switching of the beds may be triggered by the supply reservoir reaching its upper set point pressure.
In the current state of the art systems, in both the single and the double bed systems, a portion of the dried compressed air must be used for purging the desiccant bed and this purge air is not available for operating the brakes and other pneumatic equipment.
The performance of pressure swing air dryers is limited by the mass of purge air available on each cycle. The amount of purge air is often expressed as the purge-to-feed ratio (purge/feed ratio), which is the mass fraction or percentage of the compressed air that is used for regeneration. The minimum purge/feed ratio for most applications is normally about 15 to 25%. Increasing the purge/feed ratio improves drying performance by stripping water to a lower residual level, especially at the adsorption outlet end of the bed, producing a lower dry air dew point. The cost of increasing the purge/feed ratio and achieving lower dew point air is a reduced recovery, defined as air supplied to the brakes divided by air fed to the dryer, or equivalently, 1−(purge/feed). The higher the purge/feed ratio, the lower is the air recovery and the higher is the cost of operating the compressed air system in terms of fuel consumption. Also, the higher the purge/feed ratio, the larger is the compressor needed to supply a given air demand.
The compressor runs intermittently in cycles determined by the filling and depletion of the air in the system reservoirs. The compressor pumps until the pressure in the supply tank reaches the cut-out pressure (the upper set point), for example, 896 kPa (130 psig). Then the compressor rests until the pressure in the supply tank falls to the cut-in pressure (the lower set point), for example 690 kPa (100 psig). The lower set point is reached due to air usage by the brake systems, by other pneumatic equipment, and the regeneration (purging) of the dryer.